It is now common in the art of security printing, especially in the context of the production of banknotes, to apply protective layers of varnish on security documents with a view to increase their life time.
Varnishing of banknotes was and is especially carried out to increase the durability of banknotes put into circulation. Information about the varnishing of banknotes can for instance be found in the following papers:
[Buitelaar1999]:                Tom Buitelaar, De Nederlandsche Bank NV, Amsterdam, the Netherlands, “Effects of Banknote varnishing”, Currency Conference CSI, Sydney 1999;        
[deHeij2000]:                Hans A. M. de Heij, De Nederlandsche Bank NV, Amsterdam, the Netherlands, “The design methodology of Dutch banknotes”, IS&T/SPIE's 12th International Symposium on Electronic Imaging, Optical Security and Counterfeit Deterrence Techniques III, San Jose, Calif., USA (Jan. 27-28, 2000), Proceedings of SPIE vol. 3973, pp. 2-22;        
[Wettstein2000]:                Frank Wettstein, Cash Division, Swiss National Bank, Berne and Hubert Lieb, Environmental Unit, Swiss National Bank, Zurich, “Life cycle assessment (LCA) of Swiss banknotes”, Quarterly Bulletin 3/2000 of the Swiss National Bank, September 2000;        
[Buitelaar2003]:                Tom Buitelaar, De Nederlandsche Bank NV, Amsterdam, the Netherlands, “Circulation Fitness Management”, Banknote 2003 Conference, Washington D.C., Feb. 3, 2003;        
Further information about the varnishing of banknotes and like security documents might be found in European patent application Nos. EP 0 256 170 A1, EP 1 932 678 A1 and International application Nos. WO 01/08899 A1, WO 02/094577 A1, WO 2004/072378 and WO 2006/021856 A1.
International application No. WO 02/051638 A1 discloses a flexographic printing press suitable for carrying out varnishing on the recto and verso sides of security documents, such as banknotes. This flexographic printing press is in particular characterized in that flexographic printing units are disposed above and below the path of the sheets being varnished. Recto-verso varnishing is thus carried out without this necessitating reversal of the sheets during processing thereof.
Japanese patent application No. JP 2007-176044 A discloses a recto-verso offset printing press equipped with a varnishing system for recto-verso varnishing of the printed sheets. European patent application No. EP 1 880 845 A2 similarly discloses a recto-verso offset printing press equipped with coating units for recto-verso varnishing of the printed sheets.
The varnishing of banknotes is also commonly performed on so-called polymer banknotes, which polymer banknotes are characterized by an all-plastic transparent polymer substrate (typically a BOPP—biaxially oriented polypropylene—laminate) with white opacifying layers provided on both sides thereof.
Varnishing has demonstrated its value as an efficient way of increasing the durability and life time of banknotes and like security documents. Nevertheless, it has been noticed that varnished banknotes were still prone to soiling and that the varnishing process still needs to be improved.
It has in particular been noticed that intaglio-printed security documents, even though varnished on both sides, still needed to be improved in terms of their resistance to soiling. Furthermore, it has been noticed that intaglio-printed security documents were more likely to get soiled on one side than on the other.
Intaglio printing is a well-known printing process that is especially used for the production of security documents. Information about intaglio printing and intaglio printing presses for carrying such a printing process may be found in Swiss patents No. CH 289 716, CH 373 770, CH 477 293, German patent DE 1 058 074, European patent applications Nos. EP 0 091 709 A1, EP 0 406 157 A1, EP 0 415 881 A2, EP 0 873 866 A1, and International application Nos. WO 03/103962 A1, WO 2005/077656 A1 and WO 2005/118294 A1, all in the name of the present Applicant.